Cutting tools having been widely used to cut metals are typically covered with a multilayer coating that are formed by depositing a TiCN layer, an Al2O3 layer and other layers on the surface of a base made of a cemented carbide or the like. Also, it is known that the heat resistance of a cemented carbide is increased by adding a carbide other than WC, such as TIC, TaC, or NbC to a cemented carbide. It is, however, also known that the concentration of carbides other than WC, such as TIC, TaC, and NbC, that is, β phase concentration, is reduced at the surface of the cemented carbide.
Japanese Unexamined Patent Application Publication No. 8-118108 discloses a cutting tool in which the surface of a cemented carbide base is covered with a TiN layer, a TiCN layer, a TiC layer, a TiCNO layer, an Al2O3 layer and a TiN layer in that order by CVD (chemical vapor deposition). In this disclosure, W and Co are diffused in the grain boundaries of the TiN, TiCN and TiC layers that are closer to the base.
Japanese Patent No. 4253184 discloses a method. For forming a B—C-containing coating, in this method, an amorphous coating of Nb or any other metal is formed by unbalanced magnetron sputtering, and an intermediate layer in which the content of the Nb or any other metal is gradually varied is made by gradually increasing the electric power applied to a target containing B and C while gradually reducing the electric power applied to the Nb or any other metal coating.
The following is a list of the aforementioned background art
PTL 1: Japanese Unexamined Patent Application Publication No. 8-118108
PTL 2: Japanese Patent No. 4253184